chemical physics of discharges - Argonne National Laboratory

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have observed ions having the general formula, H+(H O)n, with n varying from 1 to 7,
and Wexler, et al. (46) have observed polymer ions Prom acetylene having up to 12
carbon atoms.
Recently, studies have been carried out in mass spectrometry sources at pressures
up to several torr. Under these conditions, any primary ions formed will undergo
many collisions and will have ample opportunity to react if they are capable of
doing so. Field and Munson (47,48) have taken advantage of this to carry out some
very intereciting studies of reactions of highef order. pey observeh that in very
pure methane, the principal secondary ions CH5 and C2H5 reached a plateau and re-
(47) M.S.B.Munson and F. H. Field, 2. &. =m. %. 88, 2621 (1966)
(48) F. ti. Field, M. S. B. Munson and D. A. Becker, "Ion Molecule Reactions in
the Gas Phase," Adv. in Chem. Series, Amer. Chem. SOC., Washington, D. C.
(49)
(1966), p. 167
P. Kebarle and A. M. Hogg, 2. s m . m. 42, 798 (1965)
(50). A. M. Hogg and P. Kebarle, J. Chem. Phys. 43, 498 (1965)
mained constant with increases in pressure beyond about 0.5 torr. They observed
also that if there were any small amount of impurities in the methane the intensities
of these ions passed through a maximum around a few .loth8 torr and then declined
steadily with further increases in pressure. The primary ions had very small probability
of colliding with anything but methane and consequently the disappearance
of the secondaries must be due to their reaction with the impurities, since it is
demonstrated that they did not react with methane itself. It was a simple step
from this to the addition of small amounts of a variety of materials to the methane
plasma, with results that proved extremely interesting. When, for example, smell
amounts of long chain paraffin hydrocarbons, such as dodecane, were added to the
methane plasma, the spectrum of ions from the high molecular weight paraffin was
quite different from that obtained by electron impact. Such high molecular weight
paraffins give only small intensities of ions above about the C5 range under
eleqtron impact. However, in the methane plasma, ions of the general composition
C H formed at each carbon number from the parent down to C4. No doubt ions
o? tailer mess are also formed, but these are not observable because of the interference
from secondary and ternary ions from methane. Further, the largest of thfse
ions is the one corresponding to the parent molecules; i.e., with dodecane,C12H25 .
Field and Munson have studied a number of compounds by this method, and in
many instances have obtained profound changes in the mas spectrum, produced by
"Chemical Ionization," the term which they have given the processes. (47,48)
They have concluded that in the case of the methane the principal reactions are
probably as follows:
CH5 + + CnH2w2 + CH4 + H2 +'Cn+H + ,
2#1 .
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+
4 C H + CnH2#l
2 6
Although the previous discussion has been devoted entirely to reactions of positive
ions, negative ions are also known to undergo reactions on collision. Relatively
few of these reactions have been studied, however, largely 'because negative
ions present some rather serious difficulties to the investigator.
of negative ions, however, have been carried out by Melton, Henglein and others,
and several typical reactions are given in table 9.
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c
Some reactions